refactor: move nix experimental feature setup to nix.conf and rename pkgmgr wrapper

https://chatgpt.com/share/693dcbad-3d30-800f-acfe-22f7263f3e80

scripts/launcher.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+FLAKE_DIR="/usr/lib/package-manager"
+
+# ---------------------------------------------------------------------------
+# Try to ensure that "nix" is on PATH (common locations + container user)
+# ---------------------------------------------------------------------------
+if ! command -v nix >/dev/null 2>&1; then
+  CANDIDATES=(
+    "/nix/var/nix/profiles/default/bin/nix"
+    "${HOME:-/root}/.nix-profile/bin/nix"
+    "/home/nix/.nix-profile/bin/nix"
+  )
+
+  for candidate in "${CANDIDATES[@]}"; do
+    if [[ -x "$candidate" ]]; then
+      PATH="$(dirname "$candidate"):${PATH}"
+      export PATH
+      break
+    fi
+  done
+fi
+
+# ---------------------------------------------------------------------------
+# If nix is still missing, try to run nix/init.sh once
+# ---------------------------------------------------------------------------
+if ! command -v nix >/dev/null 2>&1; then
+  if [[ -x "${FLAKE_DIR}/nix/init.sh" ]]; then
+    "${FLAKE_DIR}/nix/init.sh" || true
+  fi
+fi
+
+# ---------------------------------------------------------------------------
+# Primary path: use Nix flake if available
+# ---------------------------------------------------------------------------
+if command -v nix >/dev/null 2>&1; then
+  exec nix run "${FLAKE_DIR}#pkgmgr" -- "$@"
+fi
+
+echo "[launcher] ERROR: 'nix' binary not found on PATH after init."
+echo "[launcher] Nix is required to run pkgmgr (no Python fallback)."
+exit 1